1. Field of The Invention
This invention relates to detection and measurement of losses in a branched optical fibre network.
2. Related Art
It is well known that faults in optical fibres can be located by an OTDR (optical time domain reflectometer). An OTDR launches a pulse of light into a fibre, and backscattered light is monitored for abrupt changes indicative of a fault, the time between pulse launch and the detection of the light at the launch end being indicative of the distance along the fibre that the fault occurs.
In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable for example of being transmitted by dielectric optical waveguides such as optical fibres.
In branched networks, it would be extremely costly to monitor each line separately from the most diversified end. It is, therefore, desirable to be able to monitor the branch lines from a centralised location, such as an exchange. If an OTDR is used, then the backscattered light from each branch line is combined on its return to the junction of the branches, and it is not possible to determine from which branch line it originated, although a distance from the pulse source is known. Also, in a branched network, the power of the outbound pulse is divided into the branch lines. Thus, the information relating to any branch line has only the optical power resulting from the backscattering from the portion of the pulse in that branch line superimposed on the information from all the other branch lines, which will decrease the resolution thereby reducing the dynamic range of the instrument and sensitivity of attenuation measurement in a particular branch line. In general, an OTDR at present has a backscatter range limitation of approximately 20 dB for a 100 ns pulse width. Thus, if the branch lines are of substantial length and/or diversity, it may not be possible to monitor the entire network by normal OTDR methods. In any event, specific branch line(s) at fault would not be identified.
The specification of our co-pending International patent application No. GB89/01454 describes a method of detecting loss in a branched optical fibre network comprising a first optical fibre and a plurality of second optical fibres each of which is coupled to the first optical fibre, the first optical fibre constituting a main line and the second optical fibres constituting branch lines, a respective reflector being associated with each of the branch lines, the method comprising the steps of launching a pulse into the main line and monitoring the main line for changes in attenuation of reflected signals returning from the reflectors.
Although this method does work satisfactorily in many branched networks, it does have disadvantages which prevent its use in all such networks. A time domain reflectometry technique has a spatial resolution that is proportional to the temporal width of the launched pulses. As a consequence, the method requires that the reflectors are spaced from the OTDR which launches pulses into the main line by distances which differ by more than about 20 to 30 m. This spatial resolution problem also restricts the number of reflectors that can be positioned in a given branch line, and the number of branch lines that can be effectively monitored. Another disadvantage of this method is that, if (as is preferred) the reflectors reflect light at different wavelengths, the loss detection signals take up a significant part of the optical spectrum, thereby restricting the amount of the spectrum available for traffic.